This invention relates to the production of tubular elements such as pipes or tanks, especially such tubular elements as are formed of fiber reinforced thermosetting resin materials and which are provided with a lining particularly in the form of a thermoplastic resin lining.
As is known, fiber reinforced thermosetting resin materials are well suited to the manufacture of tubular elements from the standpoint of structural strength, but as is also well known, the fiber reinforced thermosetting resin materials have a characteristic tendency to "weep" or leak through the wall structure, particularly where a substantial pressure differential exists as between the inside and the outside of the pipe or tank. This weeping or porosity is so severe in some cases as to preclude use of the pipes or tank for many purposes for which the fiber reinforced thermosetting resin is otherwise well adapted.
In view of this tendency to weep, it has been proposed to apply an impervious lining to tubular fiber reinforced thermosetting resin elements and this has been done by a variety of techniques in which the lining is made of thermoplastic resin material. However the techniques heretofore employed for this purpose are subject to one or more of a number of disadvantages. Generally these disadvantages fall into several categories, including processing difficulties, excessive production costs, and incompatibility of the thermoplastic resin with the thermosetting resin so that the lining is not effectively bonded to the body of the tubular element.
More specifically the prior techniques include the following:
In some cases after completion of an unlined tubular thermosetting resin element a lining has been applied to individual pieces of the tubing by centrifugal or tumbling coating operations. Techniques of this kind are disadvantageous because of excessive production costs resulting from the piece-by-piece handling required. In addition, this type of internal coating does not ordinarily provide a secure bond between the thermoplastic lining and the thermosetting resin of the body of the tubular element.
In another known technique for applying a lining, the lining was first preformed, as by extrusion or molding and the preformed lining tube was then placed in a filament winding machine in which the fiber reinforced resin laminate was applied in superimposed layers. This prior technique also has the major disadvantage of excessive and expensive piece-by-piece handling and also the disadvantage that an effective bond between the thermoplastic and thermosetting resins is not provided.
Some attempts have also been made to apply a lining layer upon a mandrel, for instance by helically winding a strip of thermoplastic resin material, winding the fiber reinforced thermosetting resin on top of the lining layer and advancing the tubular structure being built in this way axially along the external surface of the mandrel, i.e., by sliding the lining layer on the surface of the mandrel. This technique is subject to various processing difficulties, including the fact that sliding motion of the lining on the mandrel has a tendency to disrupt the integrity of the lining especially if the thermosetting resin laminate is heated to effect curing thereof while it is on the mandrel, such heating naturally tending to soften the thermoplastic resin of the lining layer.
In contrast to all of the foregoing, the present invention provides a distinctive technique in which the thermoplastic lining layer is first applied to an axially moving mandrel surface, the thermosetting resin laminate is built upon the lining layer, and the axial motion of the mandrel surface carries all layers of both the lining and the body of the tubular element in an axial direction without any sliding motion of the lining layer on the mandrel. By operating in this way, continuous and high speed production is readily attainable, and the piece-by-piece handling of finite lengths of the tubular element is eliminated.
The invention further contemplates special provision for the bonding of the thermoplastic lining layer to the thermosetting resin of the fiber reinforced laminate superimposed upon the lining layer, thereby overcoming the difficulties heretofore encountered because of lack of effective bonding between the layers, such as the tendency for the lining to break away from the laminate and disintegrate within the tubular element.
Still further the invention contemplates employment of a sequence of operations according to which the lining layer may readily be formed in situ on the axially moving surface of the mandrel. In this way bulk material for formation of the thermoplastic lining may continuously be fed to the mandrel and the lining may be progressively fused from the applied bulk material in advance of the zone of application of the fiber reinforced thermosetting laminate. Continuous application of a bonding means is also provided for in advance of the zone where the laminate is applied and in this way an overall production system is provided requiring a minimum of attendance.
In accordance with another aspect of the invention, provision is made for continuously applying an impervious, preferably a thermoplastic, jacket layer superimposed upon the fiber reinforced thermosetting laminate, this jacket layer also being applied while the lining layer and the laminate are carried by and are being advanced with the axially moving surface of the mandrel. Provision is also made for effecting secure bonding of the jacket layer to the thermosetting resin laminate notwithstanding the use of impervious thermoplastic material for the jacket layer.